In recent years, along with an increase in awareness of energy saving, there have been proposed a large number of permanent magnet motors using rare-earth permanent magnets having high coercivity in a rotor to realize high efficiency. However, the rare-earth permanent magnets are expensive, thus leading to increase in cost of the motor. Therefore, in a rotor of a related-art general interior permanent magnet motor, sintered ferrite magnets are used instead of the rare-earth permanent magnets. When the sintered ferrite magnets are used instead of the rare-earth permanent magnets as described above, a residual flux density, which represents a magnitude of a magnetic force, is reduced to about ⅓. Therefore, the area of a surface of each of the permanent magnets is increased to the extent possible to compensate for lack of torque due to the reduction in magnetic force. Further, a plurality of magnet insertion holes configured to embed the plurality of permanent magnets therein are formed in a rotor core. In a core portion on a contour side with respect to each of those permanent magnets, slits extending in a radial direction are formed so as to reduce an electromagnetic exciting force generated in the motor.
For example, in Patent Literature 1, the following rotor of the interior permanent magnet motor is disclosed. The rotor of the interior permanent magnet motor includes a laminated core and a shaft. The laminated core includes a plurality of arc-shaped permanent magnets and a plurality of punched holes configured to receive the permanent magnets therein. Each of the plurality of punched holes is formed for one pole. Further, each of the plurality of punched holes is arranged so that the convex portion side of the arc is directed toward the rotor center.
Further, in the interior permanent magnet motor disclosed in Patent Literature 2, in order to increase the magnetic-path resistance against a reaction magnetic flux from a stator, in a portion of the core, which is located at an outer periphery of each of the permanent magnets, a plurality of elongated slits extending substantially in a normal direction are formed to be arrayed in a direction perpendicular to the normal. Further, when viewed substantially in a direction perpendicular to the normal, an interval between each of the slits and an outer peripheral surface of the rotor core and an interval between each of the slits and the permanent magnet are set smaller than an interval between the slits and an interval between magnetic pole pieces of adjacent teeth of the stator.
Further, in the rotor of the interior permanent magnet motor disclosed in Patent Literature 3, a plurality of slits are formed so as to greatly enlarge a region in a magnetic pole center portion, which is formed only by a magnetic portion without slits.